I have seen plastic welding kits @ Harbor Freight but never used & not sure how it works. I think it is just a torch with maybe plastic material as welding rods. I assume you heat the plastic (your crack) to melting point & melt fresh plastic material to the area, but not sure.

Or I would research best material to patch plastic. (Epoxy, Bondo, Fiber Glass body filler, etc) Fiber glass body filler would be my guess, Epoxy may be to brittle for flexing plastic.

drill a hole at each end of the crack to keep it from spreading. then drill holes on each side of the crack about 3/4" apart use zip ties going across the crack like stitches from the doctor. wont be water tight but will hold up a long time.

drill holes on both ends, then use a bolt and a wide washer through the original holes. ill have to post a picture of it. Buy the single wheel version next time. Much more stout, and I actually like the single wheel better than the double.

Plastic welding is your best bet, just make sure your welding with the same material, we don't use wheelbarrows we use the largest extra heavy duty gorilla carts, we have three or four they keep us from having to push a wheelbarrow up steep driveways and such, and they can be towed behind the mowers also.